Ability of heart and lungs to deliver oxygen to the muscles. Again, standard coaching regurgitation is to explain how slow training builds capillaries that improve ability to deliver oxygen to the working muscles.

Of course we who practice M2 training methodology have known the above to be true for many years now. My indoor cycling classes is perhaps the broadest proofpoint, where we have had virtually every ability represented in class. Novices, Iroman professional champions, age-groupers, and heck, even a Tour de France Champion.

In any case, interesting to see mainstream thought begin to see a bit of light on more thoughtful ways to train than mindless repetition of training mantras that are at odds with real-world experience.